B chromosomes in Nierembergia aristata (Solanaceae): nucleolar activity and competition with the A chromosomes

B chromosomes are additional dispensable chromosomes that may be present in some individuals, populations, or species, which have probably arisen from the A chromosomes but follow their own evolutionary pathway. Supposedly, B chromosomes do not contain major genes except for ribosomal DNA (rDNA) sequences that have been mapped on the supernumerary chromosomes of many plants and animals. This paper is a new report of B chromosome occurrence in plants. B chromosomes with nucleolar organizing regions (NORs) were found in a diploid sample of Nierembergiaaristata D. Don (sub nom. N. stricta Miers) (2n = 2x = 16). This is an extreme case in which B chromosomes possess not only strong nucleolar activity, as revealed by conventional staining methods, AgNOR and fluorescence banding, and fluorescent in situ hybridization (FISH), but also show nucleolar competition with the A chromosomes. The observed phenomenon could be analogous to the nucleolar dominance or 'differential amphiplasty' phenomenon that occurs in interspecific hybrids.     

Breeding system of Macromeria viridiflora (Boraginaceae) and geographic variation in pollinator assemblages

This study explores the association between variation in pollinator type and flower size in Macromeria viridiflora (Boraginaceae) by studying the breeding system of the plant and the pollinator effectiveness of floral visitors. Studies were conducted at two sites where plants differ in flower size and floral visitors. Breeding system studies showed that while plants are self-compatible and occasionally produce seed autogamously, pollinators are important for reproductive success in the plants. However, plants are not pollinator-limited at these sites. Combining visitation rate and pollen deposition as measures of pollinator effectiveness, I found hummingbirds to be the most effective pollinators at both sites. Although hawkmoths also pollinate the flowers, they visit the flowers less frequently and, at one of the two sites, deposit less pollen. These results are consistent with the hypothesis that geographic variation in corolla size is the result of selection by different hummingbird species.     

Variation of pollen limitation in the early flowering Mediterranean geophyte Narcissus assoanus (Amaryllidaceae)

A recent literature review indicates that pollen limitation of female fertility is a common feature of flowering plants. Despite the ecological and evolutionary significance of pollen limitation, most studies have only examined fertility in a single population at one time. Here we investigate pollen limitation of fruit and seed set in five populations of Narcissus assoanus, a self- sterile, insect-pollinated geophyte, over 2–3 years in southern France. In common with many early spring flowering plants, pollinator visitation to N. assoanus is often infrequent. Supplemental hand-pollination of flowers with outcross pollen significantly increased overall fruit and seed set by 11% and 19%, respectively. Four of the five populations experienced some pollen limitation during the study. For a given year, there was significant variation in pollen limitation among populations. Two of the populations were pollen limited in one year but not in other years in which they were studied. Seed:ovule ratios for open- and hand-pollinated flowers averaged 0.29 and 0.33, respectively. While hand pollination significantly increased the seed:ovule ratio, the low value obtained indicates that the majority of ovules in flowers do not mature seeds despite hand pollination. The role of genetic and environmental factors governing low seed:ovule ratios in N. assoanus is discussed. Received: 28 December 1999 / Accepted: 6 April 2000     

The role of pollinator shifts in the floral diversification of Iochroma (Solanaceae)

Differences in floral traits among plant species have often been attributed to adaptation to pollinators. We explored the importance of pollinator shifts in explaining floral divergence among 15 species of Iochroma. We examined four continuously varying floral traits: corolla length, nectar reward, display size, and flower color. Pollinator associations were characterized with a continuously varying measure of pollinator importance (the product of visitation and pollen deposition) for four groups of pollinators: hummingbirds, Hymenoptera, Lepidoptera, and Diptera. A phylogenetic generalized least squares approach was used to estimate correlations between pollinator groups and floral traits across a sample of Bayesian trees using different models of trait evolution. Multivariate analyses were also employed to identify suites of traits associated with each pollinator group. We found that nonphylogenetic models typically fit the data better than phylogenetic models (Brownian motion, Ornstein-Uhlenbeck), and thus results varied little across trees. Our results indicated that species with high nectar reward and large displays are significantly more likely to be pollinated by hummingbirds and less likely to be pollinated by all groups of insects. Corolla length and flower color did not show any consistently significant associations with pollinator groups. For these two traits, we discuss alternative evolutionary forces, including phylogenetic inertia and community-level factors.     

Factors affecting pollinator movement and plant fitness in a specialized pollination system

The rate of pollen exchange within and among flowers may depend on pollinator attraction traits such as floral display size and flowering plant density. Variations in these traits may influence pollinator movements, pollen receipt, and seed number. To assess how floral display size and flowering plant density affect parameters of pollinator visitation rate, pollen receipt per flower, seed number per fruit and the between-plant pollinator movements, we studied the self-incompatible plant, Nierembergia linariifolia. Per-flower pollinator visitation rate and bout length increased linearly with increasing floral display size. Pollen receipt per flower increased linearly with increasing flowering plant density. For seed number per fruit, a polynomial model describing an increased seed number per fruit at low density and a decreased seed number per fruit at high density provided a significant fit. Per-flower pollinator visitation rate was not associated with pollen receipt per flower and seed number per fruit. Bees visited plants located near to the center of the population more frequently than plants located at the periphery. Increases in both floral display size and flowering plant density led to an increased chance of a plant being chosen as the center of the pollinator foraging area. These results suggest that even though large floral displays and high flowering plant density are traits that attract more pollinators, they may also reduce potential mate diversity by restricting pollen movement to conspecific mates that are closely located.     

Female reproductive success in gynodioecious Thymus vulgaris: pollen versus nutrient limitation and pollinator foraging behaviour

Gynodioecy is a dimorphic breeding system in which female individuals coexist with hermaphroditic individuals in the same population. Females only contribute to the next generation via ovules, and many studies have shown that they are usually less attractive than hermaphrodites to pollinators. Several mechanisms have been proposed to explain how females manage to persist in populations despite these disadvantages. The ‘resource reallocation hypothesis’ (RRH) states that females channel resources not invested in pollen production and floral advertisement towards the production of more and/or larger seeds. We investigated pollination patterns and tested the RRH in a population of Thymus vulgaris. We measured flower display, flower size, nectar production, visitation rates, pollinator constancy and flower lifespan in the two morphs. In addition, we measured experimentally the effects of pollen and resource addition on female reproductive success (fruit set, seed set, seed weight) of the two morphs. Despite lower investment in floral advertisement, female individuals were no less attractive to pollinators than hermaphrodites on a per flower basis. Other measures of pollinator behaviour (number of flowers visited per plant, morph preference and morph constancy) also showed that pollinators did not discriminate against female flowers. In addition, stigma receptivity was longer in female flowers. Accordingly, and contrary to most studies on gynodioecious species, reproductive success of females was not pollen limited. Instead, seed production was pollen limited in hermaphrodites, suggesting low levels of cross‐pollination in hermaphrodites. Seed production was resource limited in hermaphrodites, but not in females, thus providing support for the RRH. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 395–408.     

Flower morphology and pollinator dynamics in Solanum carolinense (Solanaceae): implications for the evolution of andromonoecy

Flower morphology and pollinator dynamics play an important role in the evolution and maintenance of many breeding systems, including andromonoecy. We used a series of field experiments to test the functional relationship between flower morphology and pollination dynamics (e.g., pollen receipt and export) in Solanum carolinense. We find that long-styled flowers serve primarily as pollen recipients and short-styled flowers as pollen donors, making this the first study to support the male-female interference hypothesis for the evolution of andromonoecy. However, this difference in the primary male or female function of the flowers depends on the pollinator identity. In flowers visited by Bombus impatiens, style length has a positive relationship with pollen deposition and a negative relationship with pollen removal. In contrast, neither morphological nor behavioral traits determine pollen deposition or removal by small halictid bees. We demonstrate that different pollinators could select for different floral morphologies, and thus, our research suggests that pollinator-specific interactions with flower morphology play an important role in the evolution and maintenance of anrdromonoecy.     

Reproductive trade-offs in a specialized plant/pollinator system involving Asplundia uncinata Harling (Cyclanthaceae) and a derelomine flower weevil (Coleoptera: Curculionidae)

The reproductive interactions of a specialized plant/pollinator system involving Asplundia uncinata Harling (Cyclanthaceae) and a derelomine flower weevil (Coleoptera: Curculionidae: Derelomini) were studied at La Selva, Costa Rica. The inflorescences of A. uncinata exhibit a suite of cantharophilous characters including a precisely synchronized anthesis, protogyny, thermogenesis, olfactory attraction of visitors via modified staminodes, and narrow interfloral entrances. The weevil pollinators use the inflorescences for feeding, mating, and oviposition. The larvae are detritivorous and develop either in the detaching staminate flowers or, at a more favorable rate, in the rotting infructescences. The rate of infructescence abortion was high and caused by low levels of pollination. Manual pollination treatments yielded significantly higher seed counts than obtained under natural conditions, and furthermore demonstrated the inflorescences' ability to reproduce via geitonogamy. In the longer term, the reproductive benefits of maintaining low levels of pollination may shift away from the weevils and towards the plants via an increase in the size of the pollinator population.     

Is the magnitude of pollen limitation in a plant community affected by pollinator visitation and plant species specialisation levels?

Pollen limitation on a plant community level has received little attention, although it might show which pollination-related traits may cause pollen limitation to vary among species. To address several central questions in plant reproductive biology, we investigated pollen limitation in 11 plant species, including visitation and specialisation levels of all species. The female reproductive success of most species within the studied plant community was not pollen limited, but a general tradeoff between seed production and seed weight occurred as a response to supplemental pollination. In contrast to general notion, we did not find that less visited species were most pollen limited. Instead, it appears that species with high visitation rates were most pollen limited. Our study provided conflicting evidence to whether specialisation levels may affect the degree of pollen limitation within the study community. We discuss these findings in the context of recent reviews on the occurrence, causes and consequences of pollen limitation in plants. In particular, we propose that, although pollen limitation is an important phenomenon, 1) the majority of species within a plant community may not experience pollen limitation at a given moment, 2) that common notions of which plant species should experience pollen limited reproductive success do not hold true in the studied plant community, and 3) that offspring quality is as likely affected by surplus pollen loads as is the number of offspring.     

Variation within and among the chloroplast genomes ofMelaleuca alternifolia andM. linariifolia (Myrtaceae)

Melaleuca alternifolia andM. linariifolia are commercially important Australian species harvested for their essential oils. Both species have relatively narrow and disjunct distributions on the central coast of eastern Australia. Variation in the chloroplast genome was assessed for eight individuals from each of twelve populations, representing the species' geographic range. Low nucleotide diversity withinM. alternifolia contrasted with high nucleotide diversity inM. linariifolia. CpDNA data are consistent with the southern population ofM. alternifolia being a hybrid population withM. linariifolia. The two species are sympatric in this region. Variation inM. linariifolia was geographically structured, with northern populations differing from southern populations by seven restriction site mutations, five length mutations and an inversion. There was no evidence of hybridisation of the cp genome of northernM. linariifolia with the partially sympatric speciesM. trichostachya. Intra- and interspecific variation in the chloroplast genomes ofM. alternifolia, M. linariifolia, andM. trichostachya indicate considerable potential for the use of intraspecific cpDNA studies in examining phylogenetic relationships in melaleucas.     

Relative impact of mate versus pollinator availability on pollen limitation and outcrossing rates in a mass‐flowering species

Plant mating systems are driven by several pre‐pollination factors, including pollinator availability, mate availability and reproductive traits. We investigated the relative contributions of these factors to pollination and to realized outcrossing rates in the patchily distributed mass‐flowering shrub Rhododendron ferrugineum. We jointly monitored pollen limitation (comparing seed set from intact and pollen‐supplemented flowers), reproductive traits (herkogamy, flower size and autofertility) and mating patterns (progeny array analysis) in 28 natural patches varying in the level of pollinator availability (flower visitation rates) and of mate availability (patch floral display estimated as the total number of inflorescences per patch). Our results showed that patch floral display was the strongest determinant of pollination and of the realized outcrossing rates in this mass‐flowering species. We found an increase in pollen limitation and in outcrossing rates with increasing patch floral display. Reproductive traits were not significantly related to patch floral display, while autofertility was negatively correlated to outcrossing rates. These findings suggest that mate limitation, arising from high flower visitation rates in small plant patches, resulted in low pollen limitation and high selfing rates, while pollinator limitation, arising from low flower visitation rates in large plant patches, resulted in higher pollen limitation and outcrossing rates. Pollinator‐mediated selfing and geitonogamy likely alleviates pollen limitation in the case of reduced mate availability, while reduced pollinator availability (intraspecific competition for pollinator services) may result in the maintenance of high outcrossing rates despite reduced seed production.     

Ecotype Variation of Methyl Eugenol Content in Tea Tree (Melaleuca alternifolia and Melaleuca linariifolia)

Methyl eugenol is a natural phenylpropanoid compound found in a wide range of plants used for food, flavouring, cosmetics, and health‐care. As a suspected rodent carcinogen, methyl eugenol may also be harmful to humans when present in significant concentrations. Consequently, its level has been restricted in some foodstuffs and cosmetics for some markets. In order to assess the potential to breed uniformly low methyl eugenol cultivars for an essential oil crop, tea tree, the source of ‘Oil of Melaleuca, terpinene‐4‐ol type’, we examine levels in individual trees (n = 30) from two geographic regions and six terpene chemotypes. Overall, methyl eugenol levels were low in this species (Mean [SD] 354 [239] ppm, n = 30), much lower than levels predicted to be of toxicological concern. Within each chemotype, there was a lack of evidence for correlations between terpenoid constituents and methyl eugenol levels. Further support for the independence of methyl eugenol and terpene biosynthesis was evident from similar mean levels in selected (Mean [SD] 586 [339] ppm, n = 12) and undomesticated Melaleuca alternifolia trees (Mean [SD] 480 [299] ppm, n = 5) with terpinen‐4‐ol type oils. By contrast, methyl eugenol level varied by geographic origin and chemotype. Trees from the upland region, where there is a prevalence of terpinolene type trees, had lower average methyl eugenol levels than trees from the coastal region, where there is a prevalence of terpinen‐4‐ol and 1,8‐cineole type trees.     

Variation in mutualisms: the spatiotemporal mosaic of a pollinator assemblage

Variation in time (annual and seasonal) and space (between- and within-population) is examined for the pollinator assemblage of Lavandula latifolia (Labiatae), an insect-pollinated, summer- flowering, evergreen shrub of Mediterranean woodlands in southeastern Spain. Lavandula latifolia is pollinated there by nearly 85 species of comparatively long-tongued Hymenoptera, Diptera and Lepidoptera. The diversity, composition and abundance of pollinators varied markedly between years. Lepidopteran taxa prevailed numerically in one year, hymenopterans in four years, and both groups had similar abundance in the remaining year. The vast majority of pollinators exhibited significant annual differences in average abundance. Only 35.7% of taxa were recorded in all of the six study years. The pollinator assemblage had marked seasonal dynamics. Important changes in species richness, abundance and composition took place over the flowering season of L. latifolia (nearly 3 months). Only 21.7% of insect taxa occurred throughout the flowering season. Lavandula latifolia populations at different distances from water courses differed broadly in the abundance and composition of pollinators. Hymenopterans dominated numerically at the water-distant sites, whereas lepidopterans prevailed in populations growing in the vicinity of streams. Only 40.7% of taxa were recorded at all of the four study populations. Within the same plant population, horizontal distances of the order of 25 m resulted in significant changes in pollinator composition. Variations at the four scales considered combine to produce, from the viewpoint of L. latifolia, a spatio-temporal mosaic of pollinators. As these differ broadly in frequency of pollen transfer, size of pollen loads deposited, and between-flower flight distance patterns, spatio-temporal variation will predictably result in inconsistent or contradictory selective pressures on the plant, thus hindering specialization in relation to particular pollinators.     

Using chromosomal data in the phylogenetic and molecular dating framework: karyotype evolution and diversification in Nierembergia (Solanaceae) influenced by historical changes in sea level

Karyotype data within a phylogenetic framework and molecular dating were used to examine chromosome evolution in Nierembergia and to infer how geological or climatic processes have influenced in the diversification of this solanaceous genus native to South America and Mexico. Despite the numerous studies comparing karyotype features across species, including the use of molecular phylogenies, to date relatively few studies have used formal comparative methods to elucidate chromosomal evolution, especially to reconstruct the whole ancestral karyotypes. Here, we mapped on the Nierembergia phylogeny one complete set of chromosomal data obtained by conventional staining, AgNOR‐, C‐ and fluorescent chromosome banding, and fluorescent in situ hybridisation. In addition, we used a Bayesian molecular relaxed clock to estimate divergence times between species. Nierembergia showed two major divergent clades: a mountainous species group with symmetrical karyotypes, large chromosomes, only one nucleolar organising region (NOR) and without centromeric heterochromatin, and a lowland species group with asymmetrical karyotypes, small chromosomes, two chromosomes pairs with NORs and centromeric heterochromatin bands. Molecular dating on the DNA phylogeny revealed that both groups diverged during Late Miocene, when Atlantic marine ingressions, called the ‘Paranense Sea’, probably forced the ancestors of these species to find refuge in unflooded areas for about 2 Myr. This split agrees with an increased asymmetry and heterochromatin amount, and decrease in karyotype length and chromosome size. Thus, when the two Nierembergia ancestral lineages were isolated, major divergences occurred in chromosomal evolution, and then each lineage underwent speciation separately, with relatively minor changes in chromosomal characteristics.     

Intraspecific variation in the pollen exine sculpture of Jaborosa runcinata Lam. (Solanaceae)

The variability of ectexine sculpture in the pollen of Jaborosa runcinata (Solanaceae) was studied with light and scanning electron microscopy. Three main ectexine variants are recognized: incomplete reticulate, a combination of incomplete reticulate and gemmate‐granular, and gemmate‐granular. A range of transient conditions were also noted between the two extremes. Apertures vary from a basically triporate‐aspidate arrangement to pollen grains which appear to have a rudimentary colpus. The sculptural variability of the ectexine, and a tendency towards a colpus, in J. runcinata are interesting as intraspecific variation does not appear to be common within Solanaceae.     

Interspecific pollinator movements reduce pollen deposition and seed production in Mimulus ringens (Phrymaceae)

Movement of pollinators between coflowering plant species may influence conspecific pollen deposition and seed set. Interspecific pollinator movements between native and showy invasive plants may be particularly detrimental to the pollination and reproductive success of native species. We explored the effects of invasive Lythrum salicaria on the reproductive success of Mimulus ringens, a wetland plant native to eastern North America. Pollinator flights between these species significantly reduced the amount of conspecific pollen deposited on Mimulus stigmas and the number of seeds in Mimulus fruits, suggesting that pollen loss is an important mechanism of competition for pollination. Although pollen loss is often attributed to pollen wastage on heterospecific floral structures, our novel findings suggest that grooming by bees as they forage on a competitor may also significantly reduce outcross pollen export and seed set in Mimulus ringens.     

Colored floral organs influence pollinator behavior and pollen transfer in Commelina communis (Commelinaceae)

Visual floral guides such as colored anthers, lines, dots, and UV-absorption patterns on petals are commonly observed in insect-pollinated angiosperms. Floral guides that are known to enhance foraging efficiency of visitors on flowers thus promote return visits (foraging facilitation hypothesis, which predicts that visitors will discriminate against flowers with inferior floral guides). In this study, we experimentally examined the hypothesis that floral guides also prevent pollen-theft behavior by floral visitors (theft prevention hypothesis), which has rarely been tested. Nectarless flowers of Commelina communis have three types of brightly colored floral organs: large blue petals, rewarding yellow anthers, and nonrewarding yellow anthers. Colored floral organs were removed artificially from plants in two natural populations of C. communis. Removal of the nonrewarding yellow anthers diminished hoverings in front of flowers and tended to reduce the number of total floral visitor landings, supporting the foraging facilitation hypothesis. Additionally, removal of the rewarding yellow anther decreased the frequency of legitimate landings on flowers and the legitimate landing-to-total landing ratio, which is consistent with the theft prevention hypothesis. The nonrewarding anthers and the rewarding yellow anthers were shown to play an important role in increasing visitor landings and orienting floral visitors toward a landing point appropriate for pollination, respectively. We also showed that the absence of yellow anthers decreased both pollen dispatch from brown anthers and receipt by stigmas in C. communis. These findings support both the foraging facilitation hypothesis and the theft prevention hypothesis.     

The most effective pollinator revisited: pollen dynamics in a spring-flowering herb

Most flowers are visited by a wide range of potential pollinators. However, their efficiency in pollen removal and deposition, and other behavioural factors affecting pollination effectiveness may greatly differ among taxa, and even individuals. Fritillary (Fritillaria meleagris L., Liliaceae) is a spring-flowering, critically endangered plant in the Polish flora, red-listed in most of the European countries of its range. Based on indirect evidence, that is, body pollen loads, visitation frequency and seasonal abundance, it is estimated that its key pollinators are queen bumblebees, but, as shown in the literature, the largest Fritillaria pollen loads are carried by solitary bees. To study pollinator effectiveness for floral visitors to F. meleagris, we performed a garden experiment, where we analysed pollen deposition and assessed pollen removal per single flower-visit in the plant. Similarly to field conditions reported in the literature, our experimental plants were serviced by nectar-seeking bumblebee queens and two taxa of solitary bees, small pollen-collecting Andrena and large, nectar-seeking Anthophora males. When “quality” component was addressed, despite the character of visits, insects from all groups deposited more pollen than was found on unvisited flowers, but they did not differ significantly from each other in pollen deposition on virgin stigmas. We also found some differences in pollen removal both within- and among-visitor species and control flowers, unfortunately due to extremely high variation of the results they were all statistically insignificant. However, when “quantity” component of insect performance was concerned, we observed that over 81 % of visits were by bumblebees. Bombus queens stayed on flowers significantly less time than small Andrena individuals (13 % of recorded visits) and equally long as Anthophora males (only 6 % of visits). We conclude that although all the visitor groups can pollinate the flowers of F. meleagris, bumblebee queens indeed proved to be the most effective pollinators of the plant, when both quality and quantity components of pollination are concerned.